The use of thermosetting resinous materials as patching compounds for damaged or dented sheet metal is well known. For example, unsaturated polyester resin compositions have been widely used for this purpose.
The unsaturated polyester resin compounds generally utilized in patching compositions comprise an unsaturated polyester, a monomeric material capable of copolymerizing with the unsaturated polyester, finely divided fillers and polymerization accelerators and promoters. Generally, the unsaturated polyester resin is dissolved in a liquid polymerizable monomer, and the filler materials are added. Just prior to use, a free radical initiator is added to the unsaturated polyester filling compounds to cure the resin to a solid material on standing.
The art teaches that the incorporation of inorganic mineral powders such as talc, clay, calcium carbonate or silica sand in a binder imparts impact and structural strength as well as workability to the resin composition. U.S. Pat. No. 3,230,184 teaches that where high strength-to-weight ratios are required, hollow glass spheres should be incorporated into the unsaturated polyester resin. U.S. Pat. No. 3,873,475 discloses incorporating a mixture of hollow microspheres and an inorganic filler powder in an unsaturated polyester resin to prepare a thermosetting resin formulation suitable as a metal patching material.
Metal surfaces of the parts to be used in automotive bodies, household appliances, etc., having dents, cracks or holes are conventionally repaired with inorganic filler modified thermosetting unsaturated polyester patching compositions in order to eliminate these imperfections. One method for eliminating such imperfections involves filling the dents with a curable patching composition, allowing patching compositions to cure and then subjecting the filled imperfections to a finishing operation such as sanding, buffing, grinding, etc., to obtain a smooth, uniform surface which blends with the metal surface being repaired so that the repair is indistinguishable from the remainder of the metal surface after a coat of paint has been applied to the surface. There is a continuing interest and desire in the metal repair industry, and particularly in the repair of automotive bodies, for a patching composition which, after application to a metal cavity or dent, hardens rapidly, and when hardened, is readily susceptible to mechanical finishing procedures such as being easily filed and readily sanded with good featherability. That is, the mechanically finished filled imperfection blends in well with the adjacent metal surfaces.
When the metal requiring repair is galvanized steel, it has been difficult to ensure adhesion of the body filler to the galvanized steel surfaces. Generally, the galvanized steel surfaces have been pretreated in order to provide the desirable adhesion of the body filler to the galvanized steel. The pretreatment generally has involved thoroughly grinding the galvanized surface to remove the zinc coating, sanding of the surface, acid etching of the surface to remove any remaining galvanized coating, neutralization of the acid, followed by cleaning and drying of the surface. In many instances, this treated surface is coated with an epoxy/urethane primer to ensure corrosion protection (since the galvanized surface has been removed) followed by light sanding of the primer. The body filler can then be applied to the pretreated galvanized steel surfaces, and the adhesion of the body filler to the surface is generally acceptable. However, the above processing which is required to prepare galvanized steel for body fillers is both time-consuming and costly, and the corrosion protection which is lost by removal of the galvanized surface provided by the primer is often not the equivalent of the corrosion protection afforded by the original galvanized surface.